A common form of such test system has a non-conducting probe board with multitudes of probe assemblies passing through a probe board. Each such probe assembly has a contact probe mounted in a receptacle. A diaphragm board is provided over the probe board with holes for accommodating the probe assemblies. The board is usually non-conductive and the holes are about 1/8" diameter so that hole edges rarely, if ever, contact probe pins or receptacle passing therethrough. A diaphragm board is suspended over the probe board on coil springs and/or other resilient supports. A circuit board to be tested is laid on the diaphragm board within a gasket seal and on supports to form a roof of an enclosed space comprising the circuit board, the edge seal, a side wall or rail of the instrument extending from and above the probe board, further sealing artifacts (such as a picture frame type of rectangular seal gasketing) and the probe board. The diaphragm board is inside that volume. An externam vacuum pump and line are connected thereto by a port in the wall. Pulling a vacuum in the volume causes a lowering of the circuit board as ambient air pressure above it presses down and firm contact between bottom face contact points of the printed circuits and component mounting positions and the probe contacts.
There is a concern in some segments of the electronics production industry that static can build up on the test fixtures at the volume and this can lead to static discharges and/or surface leakage currents, particularly under conditions of cold or dryness in the test region, where fast air flow occur due to cyclic vacuum/air release (after test) and/or where sprays or air jets for cleaning or other purposes are applied during test setup. The static can occur at any or all surfaces of the said volume. The static or other voltage source can also be on an external item, e.g. the circuit board under test or any object that the test technician holds near the fixture and lead to a sparking to or from a grounded part of the volume of such external object.
The literature and practice of the field include recognition of these problem areas and a number of cures including laminated constructions of test fixture parts to include conductive layers at the surfaces of such parts and/or conductive coating of such surfaces, such layers or coatings being connected to ground.
It is the object of the patent invention to improve the static discharge and leakage protection in testing fixtures of the type described and in other test environments.